The present invention is generally related to controlling drum pressure in a Heat Recovery Steam Generator (HRSG) that is contained in a steam cooled advanced machine gas turbine and its associated steam turbine for power generation.
In the 9H or 7H advanced machine gas turbines manufactured by General Electric, mainly steam cooling instead of air-cooling is utilized. During start-up, a sufficient flow of steam through high pressure (HP) and intermediate pressure (IP) bypass valves must be established as a permissive or threshold to cool the gas turbine before the gas turbine can be loaded beyond a minimum load. This minimum load is referred to as spinning reserve. Prior to and during spinning reserve, the gas turbine is air-cooled and must then be steam-cooled to be loaded above the spinning reserve.
In most applications, the HRSG includes three pressures of steam generation and hence three separate steam drums, high pressure steam (HP), intermediate pressure steam (IP) and low pressure steam (LP). The permissive of steam flow is established through HP and/or IP bypass valves, before the gas turbine can be steam-cooled. In order to establish the permissive of steam flow, drum pressure for the HP drum in the gas turbine must be greater than a predetermined level (i.e., a floor pressure setpoint (e.g., 720 psi)). Thus, when the bypass valves are open, HP drum pressure is maintained at the floor pressure setpoint. Floor pressure is the minimum pressure at which steam may be admitted to the steam turbine.
During a cold start-up in which the advanced machine has been off for a significant time period, HP drum pressure is low due to the HRSG connected to the gas turbine being cold and the HP drum pressure being below the floor pressure setpoint. For example, during start-up of the advanced machine, the HRSG is purged by air flowing from the unfired gas turbine through the HRSG. The gas turbine is then fired (after the HRSG purge) and loaded to spinning reserve, which allows the HP drum pressure to increase due to heated flue gas entering HRSG. Eventually, the HP drum pressure is greater than the floor pressure setpoint, and the HP bypass valves open to maintain floor pressure until the gas turbine is being on steam cooled and the bypass valves close.
When the HP drum pressure is greater than the floor pressure setpoint during start-up (e.g., during start-up after a recent shutdown), the bypass valves open until floor pressure is reached and then close to maintain pressure, for example, when the gas turbine is unfired or recently fired. Therefore, energy contained in the HP drum as steam pressure is lost as steam flows through the bypass valves, prior to the gas turbine reaching spinning reserve. Accordingly, HP drum pressure must then be re-established for providing steam after spinning reserve. Therefore, time and fuel are lost as sufficient heat in the HRSG is generated and pressure in the HP drum is increased to allow the gas turbine to be loaded beyond the spinning reserve.
Consequently, a need exists for providing a technique that prevents depressurization of a HP drum in the HRSG when HP drum pressure is above a floor pressure setpoint during start-up of the advanced machine.
A bypass valve pressure setpoint, hereinafter referred to as pressure setpoint, is set when an HP drum in a gas turbine is at its lowest energy level (i.e., lowest drum pressure) and still above a floor pressure setpoint. During this scenario, the HP drum pressure decays during an initial HRSG purge and increases after the gas turbine is fired. The HP drum pressure is measured and the pressure setpoint is set equal to the lowest HP drum pressure above the floor pressure setpoint. Therefore, the set pressure setpoint is greater than the floor pressure setpoint, thereby maintaining the energy contained in the HP drum. Furthermore, significant time can be saved, because HP drum pressure does not have to be re-established.
Other features and advantages of the present invention will become apparent with reference to the following detailed description and figures.